POZZOLANE AND SLAGS 147 and are determined by the method already described (see Limestones, Com- plete Analysis, 7 and 8). By treatment of the insoluble residue with sodium carbonate (see Limestones, Complete Analysis, 6, b), the silica of the silicates attacked by the acid is separated. The new residue is disaggregated and examined by the methods given for clays (q.v., 3-6) the silica and the differ- ent bases of the silicates not decomposed by hydrochloric acid thus being determined. Another determination of use in the evaluation of pozzolanic materials is that of the constituents of the silicates attackable by alkalies by Lunge and Millberg's method.x Pozzolana and trass contain, as active components, zeolitic silicates, especially a silicate of aluminium and sodium analogous to analcite, these being decomposed in the hot by caustic potash. Thus, by digesting 0-5 gram of the substance with 50 c.c. of 30% caustic potash solution for about 6 hours on the water-bath, diluting, filtering and deter- mining the silica and alumina in the nitrate, an indication is obtained of the technical value of the material. The experiments of the authors men- tioned above on various specimens of trass and pozzolana show that about 24-28% of silica and 11-13% of alumina pass into solution under this treatment. 2. Technical Examination of Pozzolane This includes : certain preliminary tests for detecting the presence of heterogeneous and inert material; the lime absorption test, and especially tests relating to the fineness, the absolute and apparent density, the setting and the strength. 1. Presence of Extraneous Matter.—The presence of earthy matters may be detected by shaking the pozzolana with water and allowing to settle : pozzolana free from earth deposits rapidly and leaves the liquid clear. When the pozzolana is heated with caustic potash solution, if earthy matter is present, the organic substances of the latter colour the potash solution brown, and the addition of acid then produces a brown precipitate. Further, on dry distillation, nitrogenous organic matter yields empyreumatic products, alkaline owing to the presence of ammonia; a pozzolana con- taining them will give, therefore, ammonia when heated with caustic potash solution. 2. Lime Absorption.—This test yields satisfactory results if a control test on a good pozzolana of known value is also made. A 10% sugar solu- tion is left in contact with an excess of spent lime for at least 12 hours, with frequent shaking. After nitration, the alkalinity of the solution is determined by means of N-hydrochloric acid, of which I c.c. =0-028 gram CaO. The test is made by treating 20 grams of pozzolana in a flask with 100 c.c. of the above solution, closing the flask and leaving it for two or three days, with occasional shaking; the liquid is then filtered through a dry filter and an aliquot part titrated with N-hydrochloric acid. The quantity of calcium oxide absorbed by 100 grams of the pozzolana is then calculated. 1 Zeitschr, fur angew. Chem., 1897, p. 428.(q.v., Complete Analysis, 9 and 10).